MECHANISM OF THE SULFATION OF CALCINED LIMESTONE PARTICLES IN COMBUSTION GASES

The reaction between SO2 and CaO, usually from calcined limestone, has been studied in an electrically heated fluidised bed in the presence of varying amounts (including zero) of O2. Measurements of the initial rate of uptake of SO2 were made for small batches of CaO injected into silica and fluidised by, for example, N2 + SO2 + O2. The initial rate is found to be independent of the concentration of O2. The indications are that the reaction involves a complicated mechanism, but includes {A figure is presented} together with the participation of other species, SyOn-. The rate constant for the first step is derived to be 0.020 ± 0.015 exp [ - (4570 ± 600)/T] m/s. It was also demonstrated that SO3 reacts directly with CaO in CaO + SO3 → CaSO4 with an almost identical rate constant. In practice sulphur absorption via SO3 plays a negligible role. Although O2 has no effect on the initial rate of sulphation, it can accelerate sulphation of CaO by SO2 soon after a layer of CaSO4 has been formed. Ultimate sulphur uptakes are decreased by an increase in the concentration of O2, as well as by a decrease in the concentration of SO2. In addition, this final uptake of SO2 shows a maximum at around 1023 K. Below 923 K sulphur absorption is low, because of the low diffusivity of SO2 through the product CaSO4. Above 923 K the overall conversion of a CaO particle to CaSO4 is limited by pores blocking at their entrances with product. The rôle of O2 is not totally clear, but it does appear to affect the crystal size or the number of cracks in the layer of CaSO4 produced. © 1990.